1. Field of the Invention
The present invention relates to a circuit which negatively feeds back and amplifies after carrying out photoelectric conversion. Especially, the present invention aims at a circuit which converts to a voltage signal a current signal outputted from a photo-diode and so on.
2. Related Background Art
A photo-diode (PD) outputs a current signal converted from a light receiving optical signal. Because the current signal outputted from the photo-diode is feeble, it is general to convert the current signal into a voltage signal and then to amplify the voltage signal. Such a current-voltage conversion circuit is generally called a trans-impedance amplifier (TIA).
FIG. 13 is a block diagram showing a schematic configuration of a conventional optical receiving circuit having the photo-diode 51 and the trans-impedance amplifier 52. A current signal converted by the photo-diode 51 of FIG. 13 is inputted to the trans-impedance amplifier 52 and then converted to the voltage signal, The trans-impedance amplifier 52 is constituted by an amplifier 53 and an attenuation circuit 54. An output of the amplifier 53 is fed back to an input side of the amplifier 53 via the attenuation circuit 54.
The photo-diode 51 is formed of a manufacturing process different from that of the trans-impedance amplifier 52. Because of this, the photo-diode 51 and trans-impedance amplifier 52 are separately formed of processes different from each other, and then both are connected electrically by using bonding wires and so on, and finally packaged.
A system transferring a large amount of data such as more than gigabit per second is constituted so as to derive element properties of the photo-diode 51 and the trans-impedance amplifier 52 at the most. For example, a number of transistors connected in series are provided in the inside of the trans-impedance amplifier 52. Therefore, an improvement of the element property such as an increase of an open loop gain is performed.
However, the open loop gain decreases as a frequency becomes higher. In accordance with decrease of the open loop gain, an input impedance of the trans-impedance amplifier 52 increases. Due to the impedance and an input capacitance of the trans-impedance amplifier 52, a reflection coefficient has a resonance point. At the point, a current-voltage conversion gain increases, and a gain fluctuation occurs in a used frequency band, resulting in an aggravation of electrical property.
On a packaged condition, when a voltage-to-ground capacitance, which is a determinant of a parasitic component of the photo-diode 51, parasitizes the input terminal of the trans-impedance amplifier 52, an output property of the trans-impedance amplifier 52, i.e. a gain at high-frequency band deteriorates, as a value of the voltage-to-ground capacitance becomes large. Bonding wires connecting the photo-diode 51 to the trans-impedance amplifier 52 add a parasite inductance at an input portion of the trans-impedance 52. As a result, a gain deviation and a group delay deviation become large at a high-frequency band.
An object of the present invention is to provide an optical receiving circuit that it is possible to even a gain in a broad frequency band.
In order to achieve the foregoing object, an optical receiving circuit, comprising:
a photoelectric conversion element for outputting a current signal in accordance with a light receiving optical signal;
a negative feedback type of an amplifier in which said current signal is inputted, said amplifier outputting a voltage in accordance with the current signal, and
a reflection coefficient setting circuit for setting an input reflection coefficient, said reflection coefficient setting circuit being provided between said photoelectric conversion element and said amplifier.
Furthermore, an optical receiving circuit, comprising:
a photoelectric conversion element for outputting a current signal in accordance with a light receiving optical signal;
a negative feedback type of an amplifier in which said current signal is inputted, said amplifier outputting a voltage in accordance with said current signal, and
a reflection coefficient setting circuit for setting an input reflection coefficient, said reflection coefficient setting circuit being provided between said photoelectric conversion element and said amplifier;
wherein said reflection coefficient setting circuit minifies the input reflection coefficient at a sympathetic vibrating frequency and evens a frequency dependency of a gain of said amplifier.
Furthermore, an optical receiving circuit, comprising:
a photoelectric conversion element for outputting a current signal in accordance with a light receiving optical signal;
a negative feedback type of an amplifier in which said current signal is inputted, said amplifier outputting a voltage in accordance with said current signal;
a reflection coefficient setting circuit for setting an input reflection coefficient, said reflection coefficient setting circuit being provided between said photoelectric conversion element and said amplifier; and
a plurality of bonding wires for connecting between said photoelectric conversion element and said reflection coefficient setting circuit, and between said reflection coefficient setting circuit and said amplifier, respectively.
According to the present invention, when the input impedance of a negative feedback type of the amplifier, that is, the input reflection coefficient of a sympathetic vibrating circuit equivalent to the amplifier coincides with a resonance point, a control is performed so that an absolute value of the input reflection coefficient of the amplifier becomes small. As a result, it is possible to even the gain in a broad frequency band.
Furthermore, when mounting the photoelectric conversion element and the amplifier in the same substrate, the current voltage converter sympathetically vibrates at a plurality of frequencies different from each other in the used frequency band, by using a parasite capacitance of the photoelectric conversion element, a parasite inductance of the bonding wires connecting the photoelectric conversion element and the amplifier and so on. Because of this, even if a passing S parameter S11 decreases at a high-frequency band, it is possible to cancel the decreased amount by enlarging the input reflection coefficient S11. As a result, during a broad frequency band, the gain is evened and the used frequency band is extended.